About 1 million pigs cross state lines each week destined for other farms, where they are fed or bred, said Dr. Jeffrey J. Zimmerman. That total does not include pigs sent to slaughter.

With that rate of migration, outbreak responses need to start within hours of discovering infectious diseases, he said.

With more movement of pigs, trucks, and feed, the probability of a disease transmission event increases until it "becomes a certainty," he said.

Dr. Zimmerman, a professor of diagnostic and production animal medicine at Iowa State University College of Veterinary Medicine, said growth in pig herd sizes over the past several decades also has made the herds more vulnerable to diseases.

"Those herds of 50 or 100 animals--you could achieve solid herd immunity, and the virus disappears."

Dr. Zimmerman delivered his analysis during a late February lecture at the American Association of Swine Veterinarians' annual meeting, which included lectures and lecture series on disease and biosecurity topics. The swine industry has endured deadly
outbreaks including porcine epidemic diarrhea since 2013, watched as poultry producers fought an influenza outbreak during 2015, and kept vigil for entry of foot-and-mouth disease and other diseases circulating in swine of trading partners.

Dr. Egan Brockhoff, a partner at Prairie Swine Health Services in Red Deer, Alberta, said that, without question, "We are living in a transboundary disease world."

Industry intensification, livestock concentration, and livestock mobility all present big stakes for North American swine producers.

Dr. Brockhoff said porcine reproductive and respiratory syndrome and Seneca Valley virus infection could disrupt commerce. African swine fever would be devastating. And he questioned how swine veterinarians would respond if Nipah virus emerged in North
American pigs.

He wants improved attitudes toward biosecurity on farms and improved communication among farms. Success in disease control relies on shared information, and he envisions a North American information system for the swine industry as well as a swine
industry biosecurity standard for the continent.

Dr. Jill Nezworski, a veterinarian from Buffalo Lake, Minn., provided the perspective of a veterinarian who worked with poultry producers during the 2015 outbreak of highly pathogenic avian influenza, during which about 50 million poultry were depopulated or
killed by influenza. About 42 million of those were chickens.

Dr. Nezworski said poultry producers who ensured staff members paid close attention to signs of disease--more so than would be warranted under usual circumstances--fared better than they would have. Yet, she said, some companies first reacted to outbreak
signs with denial, claiming their turkeys likely had heat stress.

But the H5N2 influenza virus caused rapid deaths, she said, noting that one group of young turkeys was all but wiped out by flu before she and colleagues could depopulate.

While the outbreak was spreading, disease authorities promoted spread of the virus by depopulating flocks in the order in which those farms reported infections, rather than on the basis of infection risks such as the sizes of farms, their proximity to other farms, or
the regional poultry density.

High foot traffic between "clean" and "dirty" areas on each farm increased the risk of disease spread, despite biosecurity measures, Dr. Nezworski said. The outbreak responders also may have overwhelmed rural areas' carrying capacities for humans, and the
small numbers of, say, hotels could have created contact between people who were working at farms with infections and those working on farms without.

Poultry producers also need to address disease risks associated with live-haul trucks and be ready to stop all poultry movement, even if temporarily. Farm operators need to know that their premise is not immune to an outbreak, as passing audits does not
confer safety.

Dr. Nezworski also said more discussion is needed on how to depopulate animals more quickly, when necessary.

Dr. Rodney "Butch" Baker, a former AASV president and retired senior clinician from Iowa State University, recalled that, soon after graduating from veterinary school, he visited a farm with an unusual odor and spread transmissible gastroenteritis from that
farm to three others that he visited the same day. Swine veterinarians have made progress since then, but they still need more work on biocontainment, he said.

Keeping pathogens off farms requires teaching people and having them buy into biosecurity, with long-term acceptance needed for it to work. He noted that some farms require temperature readings of all employees and visitors, and farms should require that
people don shoe covers before exiting their vehicles.

Weaknesses in biosecurity for transportation remain Dr. Baker's largest concern in preparing for FMD. He said that, when a truck that hauls pigs enters farm property, the farm could require that the drivers remain in the trucks. But he also noted that existing
truck washing services can cost $500, explaining why some trucks running pigs to slaughter plants will go a week without cleaning more thoroughly than sweeping.

Dr. Baker also cited his own interactions with customs officials in saying that concerns about entry of fruits and vegetables seem to be more prominent than concerns about animal diseases, and he said the U.S. has minimal disease protection at borders.

Dr. Zimmerman said a typical swine farm in the 1980s had fewer than 200 pigs, contrasted with the mean herd size today of at least 3,000 pigs.

The U.S. Census of Agriculture's hog inventory backs Dr. Zimmer­man's estimate. Figures from the 2012 census indicate that, among farms with at least 25 pigs, the mean herd size was about 3,050 pigs. When every farm with at least one pig is counted, the
mean was about 1,000 pigs.

He also said swine farms shipped about one-eleventh as many pigs across state lines to other farms in 1980 as they did in 2015.

Citing a study on persistence of influenza in pig herds (J R Soc Interface 2016;13:20160138), Dr. Zimmerman said in a herd of at least 3,000 pigs, influenza may be able to circulate indefinitely.

High turnover rates within herds, such as 200 percent turnover rates in finishing barns and 40 percent in sow herds, work against herd immunity, he said. Large populations have continuous disease circulation, with bigger and more frequent outbreaks, he said.

"We're going to need more and better vaccines if we're ever going to get on top of herd immunity," he said.